Background: The cysteine protease inhibitor Cystatin C (CstC) is linked to many diseases in both quantity and quality.Low extracellular CstC is associated with elevated protease activity that destructs the vascular walls, whereas abnormal formation of non-inhibitory CstC dimers and aggregates by a mechanism known as domain swapping can cause amyloid angiopathy.Despite its strong association with the vascular diseases, little is known about the regulation of CstC production, dimerization and secretion.